This invention relates to a method of making a manganese oxide-based material.
A variety of different manganese oxide-based materials have been suggested for use in lithium cells or lithium ion cells as an insertion material for use in the cathode. There appear to be four different oxides with stoichiometry LiMn2: lithiated spinel Li2Mn2O4 with tetragonal symmetry; orthorhombic LiMnO2 which may be synthesized at high temperature (for example by reaction of lithium oxide with Mn2O3 under argon), but which does not have good electrochemical activity; orthorhombic LiMnO2 produced at low temperatures, which appears to have reversible electrochemical activity; and a layered LiMnO2 which is said to have reversible electrochemical activity.
For example orthorhombic LiMnO2 is described in an article by Koetschau et al. (J. Electrochem. Soc., Vol. 142, No. 9 (1995), p. 2906) and is reported to convert to spinel LixMn2O4 during the first removal of lithium; this orthorhombic oxide was used in a cell as cathode material, with carbon (in the form of mesocarbon microbeads) as the anode material, and was found to cycle. The orthorhombic oxide and its conversion to the spinel form is also described in an article by Gummow et al. (Mat. Res. Bull., Vol. 28 (1993), p. 1249). Doeff et al. describe the formation of an orthorhombic sodium-based oxide, Na0.44Mn2, in which the sodium ions can undergo ion exchange with lithium ions, forming a lithiated oxide which is stable and can undergo electrochemical cycling (see J. Electrochem. Soc., Vol. 143 (1996), No. 8, p. 2507). LiMnO2 in a layered monoclinic structure is described in WO 97/26683 (Bruce et al.), this material being produced by forming a corresponding sodium based oxide, followed by ion exchange by contacting with a lithium salt dissolved in an alcohol (n-pentanol, n-hexanol or n-octanol); some electrochemical cycling of this layered oxide was possible.
According to the present invention, there is provided a method of making an oxide LiQxMn(1-x)O2, in which Q represents a transition metal and x is less than 0.5, with a layered monoclinic structure, the method comprising:
(a) synthesizing NaQxMn(1-x)O2 with a layered structure similar to that of xcex1-NaFeO2 by reacting stoichiometric amounts of a sodium salt, a manganese (II) salt, and a salt of the metal Q in solution so as to form a precipitate, drying the precipitate in air, heating the dry precipitate to a temperature of between 650 and 720xc2x0 C. in air, and then cooling the precipitate to room temperature in air; and
(b) subjecting the NaQxMn(1-x)O2 to ion exchange with a lithium salt in solution in a non-alcoholic organic solvent at a temperature between about 140 and 210xc2x0 C.
Manganese might be the only transition metal, i.e. x=0. Alternatively the oxide may contain two (or more) transition metals, by replacing some of the manganese with for example cobalt or nickel. The rapid cooling of the sodium-based oxide may be brought about by performing the heating process with the precipitate in a dish of a good heat conductor (such as silver), and by then removing the dish containing the precipitate from the furnace and placing it in contact with a large metal block at room temperature. This rapid cooling has been found to produce a product with a significantly improved crystal structure.
The ion exchange process is desirably performed under reflux, using a solvent which boils in the specified temperature range. For example the solution might be of lithium bromide in dimethyl acetamide (DMA), or in N-methyl pyrrolidone (NMP). There is desirably a considerable excess, for example a five-fold excess, of the lithium salt with respect to the sodium-based oxide to ensure that the ion exchange process goes substantially to completion, so that all the sodium is replaced by lithium.